1. Field of the Invention
The present invention relates to a coil assembly, particularly a coil assembly used as a transformer, an inductor or other components.
2. Description of the Related Art
A conventional coil assembly is shown in FIG. 10. Specifically, a coil assembly 41 includes a core 42 and a wire 43 wound around the core 42. The core 42 has a rectangular base. The wire 43 has ends connected to respective external terminals 44 which, in turn, are attached to the rectangular base of the core 42.
FIG. 11 shows another coil assembly known in the art and made from stacked green sheets. Specifically, a coil assembly 51 includes stacked green sheets made of a magnetic material, and a plurality of coil conductors 52a, 52b, 52c, and 52d provided on the surface of the stacked green sheets. Two external electrodes 56 and 57 are attached to the green sheets after the green sheets are integrally sintered. The green sheets include via holes through which the coil conductors 52a to 52d are electrically connected in series to form a coil 52.
FIG. 12 also shows a conventional coil assembly 61 which includes a coil 62 composed of stacked green sheets and including coil conductors, and two magnetic cores 63 and 64 between which the coil 62 is sandwiched. The green sheets are made of an insulating material and have no magnetic material.
Of these, the coil assembly 41 shown in FIG. 10 is low in productivity and is costly since it is necessary to wind the wire 43 around each core 42. It is also necessary to solder or otherwise electrically connect the wire 43 and the external terminals 44 together because these two parts are made of different materials. This connection is required for each coil assembly. The coil assembly 41 can not be made into a compact arrangement since it is difficult to handle the core 42 and the wire 43 if they are small.
The coil assembly 51 shown in FIG. 11 is high in productivity and is easy to handle as the coil conductors 52a to 52d can be formed by a printing or photolithographic process and major parts can be incorporated into the board. The coil assembly 51 can also be made into a compact arrangement since the coil conductors 52a to 52d are thin and fine. However, the electric property of the coil assembly 51 varies based on the degree of contraction of the magnetic green sheets during sintering process. Also, a closed magnetic path is formed in each of the magnetic green sheets, thereby deteriorating the electromagnetic property of the coil 52. Particularly, when the coil assembly 51 is used as a transformer, such closed magnetic path weakens the electromagnetic coupling between the coils and deteriorates the performance of the coil assembly.
The coil assembly 61 provides a better electromagnetic property as the insulating green sheets forming the coil 62 have no magnetic material. This assembly, however, requires a separate effective magnetic path. It is thus necessary to provide the magnetic cores 63 and 64. This results in a reduction in the productivity of the coil assembly 61. It is known to form a magnetic path by printing or otherwise applying a resin mixed with magnetic powder to encase the coil 62, each of the thus formed magnetic cores 63 and 64 having rugged mating surfaces. The resulting resinous material has a magnetic permeability one hundredth to one ten thousandth that of the magnetic material per se and can not provide an effective magnetic path.